1. Field of the Invention
The present invention relates to a mask, and more particularly, to a mask used for fabricating a polysilicon layer.
2. Description of Related Art
Generally speaking, electronic devices require switches for driving the devices. For instance, an active display device is often driven by a thin film transistor (TFT) acting as the switch. The TFTs can be classified into amorphous silicon TFTs and low-temperature polysilicon TFTs (LTPS-TFTs) based on materials of channel regions of the TFTs. By virtue of relatively low power consumption and great electron mobility in comparison with the amorphous silicon TFTs, the LTPS-TFTs have drawn more attention in the industry. Hence, low-temperature polysilicon crystallization technologies are extensively developed, wherein a sequential lateral solidification (SLS) laser crystallization technology is the mainstream of the crystallization technologies.
FIG. 1 is a schematic view of a conventional SLS laser crystallization apparatus 100. Referring to FIG. 1, the SLS laser crystallization apparatus 100 includes a laser light source (not shown), an optical system 110, and a substrate carrier 120. A substrate 130 on which an amorphous silicon layer (e.g., α-Si depicted in FIG. 1) is formed is placed on the substrate carrier 120. The substrate 130 is often laterally shifted by the substrate carrier 120 at a distance on a scale of millimeters. First, a laser beam 140 passing through a mask 112 is patterned, and the laser beam 140 irradiates the amorphous silicon layer on the substrate 130 through a projection lens 114 with a zoom-in ratio, a zoom-out ratio, or a proportional zoom ratio. Slits of the mask 112 restrict regions irradiated by the laser beam 140. Therefore, the amorphous silicon layer irradiated by the laser beam 140 is transformed to be silicon which is “melted state”, while the non-irradiated amorphous silicon layer remains solid state. The melted silicon is laterally grown with use of the amorphous silicon layer as a nucleus, such that the amorphous silicon layer in the irradiated region becomes a polysilicon layer (e.g. p-Si depicted in FIG. 1). Next, with a stepping movement of the substrate 130, the entire amorphous silicon layer is transformed into the polysilicon layer having a periodic grain arrangement by sequential lateral crystallization. Patterns of the mask 112 control the position of a grain boundary and the region where a lateral crystallization occurs, and the grain size and the crystallization quality of the polysilicon layer significantly rely on the patterns of the mask 112.
FIG. 2A illustrates a mask used in a conventional SLS laser crystallization apparatus. FIG. 2B is a schematic partial view of a polysilicon layer formed by performing the SLS laser crystallization technology with use of the mask depicted in FIG. 2A. Referring to FIGS. 2A and 2B, a mask 200 has an opaque pattern 210 for defining a plurality of strip transparent slits 220 without any pattern therein. The transparent slits 220 are arranged in array, and the transparent slits 220 arranged in even columns and the transparent slits 220 arranged in odd columns are alternately arranged and partially overlapped to one another in a row direction. A polysilicon layer 250 formed by performing the SLS laser crystallization technology with use of the mask 200 has a plurality of primary grain boundaries (PGBs) and a plurality of secondary gain boundaries (SGBs) perpendicular to the PGBs. Here, the PGBs are also referred to as main grain boundaries, while the SGBs are also referred to as sub-grain boundaries. The number of the SGBs is inversely proportional to the carrier mobility of the polysilicon layer 250. Hence, when a direction of a current passing through a channel region of a polysilicon TFT is perpendicular to the PGBs but parallel to the SGBs, the carrier mobility of the polysilicon TFT is relatively high. However, when the polysilicon layer is patterned to form the channel regions of the TFTs, electrical properties of various polysilicon TFTs are quite different, given that length directions of the channel regions in the different TFTs and the PGBs together form included angles that are not exactly the same.